Different immunoglobulin classes, complement factors, complement fixation and rheumatoid-factor activity in rheumatoid synovial membranes of 16 patients were studied. After pepsin digestion of tissue sections, greatly increased numbers of plasma cells were able to bind human gamma globulin. In addition we found after the pepsin digestion more IgG and less IgM in the tissues. The results gave evidence of intracellular blocking of rheumatoid factor activity, due to IgG-IgG rheumatoid factor complexes or self associated rheumatoid factors. Additional evidence of such plasma cell complexes was obtained by observing an in vitro fixation of the complement. With the same methods we studied 3 patients whose synovial membranes had been surgically removed not later than 6 months after the onset of evident symptoms and who developed rheumatoid arthritis within an observation period of 2 years. As a result we found a small production of predominantly M and G immunoglobulins. The rheumatoid factors were also found in a small degree. Rheumatoid factor activity can be revealed only in a little number of plasma cells by pepsin digestion of tissues. In the serum we can't find more rheumatoid factor activity after pepsin digestion in the latex test. The immunofluorescence for detection of rheumatoid factors in tissues is a practicable method and important for the differential diagnosis of joint diseases.